This invention relates to the art of nuclear reactors and in particular to the core restraint system of a nuclear reactor. A nuclear reactor includes a pressure vessel into which a heat-transfer fluid, typically sodium for fast-breeder reators, is pumped under pressure. The sodium flows through the core and is heated; the hot sodium emerges from the vessel and flows to electrical power-generating equipment. Within the vessel there are the core components. Typically these components include fuel-rod bundles or assemblies, control-rod assemblies, blanket fertile material or fertile rod assemblies and removable radial shielding assemblies. These assemblies fit into a core support structure which serves the purpose of locating, supporting and distributing coolant to the core. Surrounding and providing axial restraint for these assemblies are core restraint former rings which also provide for restraining deformation of the core.
One of the functions of the core restraint system is to prevent bowing motions in the fueled regions of the reactor assemblies which add positive reactivity of such rate and magnitude as to result in a positive power reactivity coefficient. These bowing motions usually result from lateral temperature gradients that are established in the reactor assemblies as the reactor is brought to power. Undesirable bowing motions are limited by providing assembly contact pads on the core assemblies at appropriately chosen planes. In the presently described liquid-metal-cooled fast-breeder reactor, load pads are provided each assembly just above the fissile and fertile portion of the core, called the above core load plane (ACLP), and at the top of each assembly, referred to as the top load plane (TLP). These pads assures that once interassembly gaps are closed at the load (or contact) planes, bowing of the fueled regions of the reactor assemblies is radially outward as the reactor is brought to power. The reactivity characteristics of fast reactors are such that expansion of the fueled region adds negative reactivity. Thus the outward bowing motion of the fueled region as the reactor is brought to power will enhance the negative source reactivity coefficient.
In order to facilitate reactor core refueling, some space or gaps must be present at the load planes at reactor refueling temperature, which is about 400.degree. F., to prevent undue stress on the assemblies during withdrawal and insertion. If these interassembly spaces or gaps at the load planes are not closed prior to reactor startup, bowing motions of the fueled regions of the reactor assemblies will be inward during power ascent, adding positive reactivity until the load plane interassembly gaps are closed. The significant sources of reactivity changes other than control motion in a sodium-cooled fast-breeder reactor include radial fuel and blanket assembly motion and the doppler effect. As the fuel temperature increases, the doppler effect results in the addition of negative reactivity. In a reactor in which the assembly lateral temperature gradients are radially oriented with the high temperatures on the inboard side of the assemblies, the positive reactivity insertion may exceed the negative reactivity additions and result in a positive reactivity coefficient unless assembly motions are closely controlled.